Stroke writing character generator with reduced bandwidth

ABSTRACT

A stroke writing character generator pauses between the generation of vectors, which may meet at sharp corners, so as to reduce deflection bandwidth. A stroke state machine having a plurality of states controls the generation of the vectors. The plurality of states includes a time pause state. A vector length counter provides an end of vector signal that causes the stroke state machine to enter the time pause state for inserting a time pause between the generation of the vectors. The video of the CRT on which the vectors are written is blanked in coincidence with the time pause.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to electronically generated displays particularly with respect to stroke writing character generation.

2. Description of the Prior Art

Electronic displays are known wherein the characters and symbology are electroncially drawn on a display face utilizing a series of concatinated vectors to compose the characters and symbols. Such systems typically utilize a cathode ray tube (CRT) wherein the CRT beam is deflected by the deflection system to draw the vectors on the screen. The CRT video input is energized or deenergized so as to draw the vectors or to blank the vectors. Such a display is disclosed in U.S. Pat. No. 4,553,214 issued Nov. 12, 1985 in the name of the present inventor for "Angle Based Stroke Generator " assigned to the present assignee. Said U.S. Pat. No. 4,553,214 is incorporated herein in its entirety.

In such stroke or vector writing displays, numerous characters and symbols comprise sequences of consecutively drawn straight line vectors that meet at oblique or right angles defining sharp corners. In order to execute such sharp corners, high bandwidth deflection systems are required. High bandwidth deflection systems necessitate undesirably large amounts of power, complexity and cost. If, however, a low bandwidth system is utilized the sharp corners will be rounded hindering recognition of the symbology. The display of sharp corners is required for adequate character legibility.

SUMMARY OF THE INVENTION

The present invention obviates the above discussed problems of the prior art systems by inserting a time pause between the end of one stroke and the beginning of the next stroke at a corner in order to reduce the bandwidth of the deflection system. Preferably the beam is blanked during the pause.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the stroke writing procedure for generating a letter "A".

FIG. 2a is a graph of deflection vs. time illustrating two consecutive vectors meeting at a sharp corner.

FIG. 2b is a graph of deflection vs. time illustrating the two vectors of FIG. 2a with a time duration pause therebetween.

FIG. 2c is a graph of blanking vs. time illustrating a video blanking pulse coincident with the time duration pause of FIG. 2b.

FIG. 3 is a spectral density plot of amplitude vs. frequency illustrating the conserved bandwidth when utilizing the present invention.

FIG. 4, comprised of FIGS. 4a and 4b, is a schematic block diagram of a stroke writing display incorporating the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is preferably embodied in the display system of said U.S. Pat. No. 4,553,214 which is incorporated herein in its entirety. FIGS. 4a and 4b are substantially the same as FIGS. 2a and 2b of said U.S. Pat No. 4,553,214 and the modifications thereto utilized in practicing the present invention will be described below. FIG. 1 of the present application is also substantially the same as FIG 1a of said U.S. Pat. No. 4,553,214.

Referring to FIG. 1, the letter "A" is illustrated drawn utilizing stroke writing. The generation of this character by stroke writing was described in said U.S. Pat. No. 4,553,214. Briefly, the CRT beam is deflected through a series of sequential vectors 1-6 starting at a point (X₀, Y₀). During vectors 1, 4 and 6, the beam is blanked and during vectors 2, 3 and 5, the beam is intensified. The character illustrated in FIG. 1 demonstrates the requirement for sharp corners to maximize character legibility. It is appreciated that if the corners whereat consecutively drawn vectors meet were rounded by a low bandwidth deflection system, recognition of the symbol may be hindered. FIG. 2a illustrates the deflection signal that results when the beam is required to execute a sharp oblique angle at a time t₁. The drawing of the second of the two vectors illustrated ends at time 2t₁.

Referring to FIG. 2b, the deflection signal utilized to draw the two vectors of FIG. 2a in accordance with the present invention is illustrated. The drawing of the first vector terminates at time t₁ and the drawing of the second vector begins at time t₁ +T. Therefore, the pause inserted at the corner between the two illustrated vectors is of time duration T and is denoted by reference numeral 7. FIG. 2c illustrates a blanking pulse 8 to be applied to the video input of the CRT coincident with the pause 7 of FIG. 2b. The pause 7 interposed between successive vectors in the preferred embodiment has a duration T of one microsecond.

Referring now to FIG. 3, the frequency spectra F_(A) (f) and F_(B) (f) of the deflection signals of FIGS. 2a and 2b are illustrated, respectively. These frequency spectra may be expressed as follows: ##EQU1## It is appreciated that these frequency spectra are of the form (Sin X/X)².

To quantitatively determine the bandwidth reduction benefit of the invention, the bandwidths of the two spectral density plots F_(A) and F_(B) are compared at equal energy levels. For convenience, the comparison is effected with respect to the first zero crossings of the plots which provides 92% of the total energy thereof. The pause T reduces the zero crossing frequency from 1/t₁ to 1/(t₁ +T). This is a bandwidth reduction of ##EQU2## In a specific embodiment where the average stroke length is 0.070 inches and the writing rate is 15,000 inches per second:

    t.sub.1 =0.070/15,000=4.6667 microseconds.

With a pause of T=1.0 microseconds, the bandwidth reduction is

    [1.0/(1.0+4.6667)]×100=17.65%

Thus it is appreciated in the specific embodiment discussed the one microsecond pause results in a significant bandwidth reduction. On the frequency axis of FIG. 3, one division represents (1/17)×10⁶ Hertz.

Referring to FIG. 4, comprised of FIGS. 4a and 4b, a schematic block diagram of a stroke writing display system for practicing the present invention is illustrated. As discussed above, FIGS. 4a and 4b are substantially the same as FIGS. 2a and 2b of said U.S. Pat. No. 4,553,214 wherein the structure and operation of the illustrated apparatus is described in detail. Briefly, the apparatus is controlled by the stroke state machine 26 which sequences from state to state to effect the various functions performed in displaying stroke written characters. For example, prior to writing a vector, the length of the vector is loaded into counter 90. During the writing of the vector the counter 90 is decremented as described in said U.S. Pat. No. 4,553,214. When the counter 90 attains zero count, it provides a status signal to the state machine 26 designating that the end of the vector has been attained and the next following vector may be initiated.

In practicing the present invention an additional state is incorporated into the stroke state machine 26 to effect a time pause at the end of the writing of a vector. Thus, when the stroke state machine 26 receives the status signal from the counter 90 indicating that the writing of a vector has been completed, the stroke state machine 26 sequences to the time pause state. When the predetermined time pause has occurred, the stroke state machine 26 then sequences to the initial state for drawing the next vector as otherwise described in said U.S. Pat. No. 4,553,214. While the stroke state machine 26 is in the time pause state, a control signal is applied to the blanking block 40 of FIG. 4b via a control lead 81 for blanking the video. FIG. 2c illustrates the blanking pulse applied to the lead 81.

Timing circuits for implementing the time pause state in the stroke state machine 26 are well known in the art. Preferably dummy or no operation (NOP) states may be included in the stroke state machine 26 for effecting the pause or, for example, a preset counter may be included therein for timing out the pause. When the counter counts down to zero, the stroke state machine 26 is advanced to the next state. Alternatively, a monostable multivibrator may be utilized to effect the time pause.

Although the preferred embodiment of the invention is described in terms of pausing at the end of the drawing of a vector prior to initiating the next vector, the invention may also be embodied by including the time pause state as the first state in drawing a vector rather than as the last state in the vector drawing procedure.

The present invention provides the additional benefit of minimizing the complexity and speed of the digital logic that controls the drawing of the characters. Since the time pause introduced at the junction of vectors to reduce the bandwidth requirements of the deflection system provides additional time for the digital data handling associated with the character generation, slower and hence less complex and expensive digital logic may be utilized. For example, slower memories may be employed in implementing a system that embodies the present invention.

Thus, utilizing the invention results in a simplification of the digital logic that generates the stroke commands. Whereas, the prior art increased power and bandwidth of the deflection amplifiers to follow sharp edges, the present invention pauses between strokes to reduce deflection harmonics. It is appreciated that utilizing the present invention provides consecutively draw vectors meeting at sharp angles without suffering a deflection power penalty. By digitally limiting deflection bandwidth requirements, no display quality is sacrificed while reducing deflection system power, complexity and cost.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes may be made within the purview of the appended claims without departing from the true scope and spirit of the invention in its broader aspects. 

What is claimed is:
 1. Stroke display apparatus, having CRT display means with a video input and a display face, for writing a sequence of connected vectors on said display face, each vector having a length associated therewith, comprising:a stroke state machine having a plurality of states for controlling the generation of said vectors, said plurality of states including a time pause state, vector length counter means loaded with the length of a vector to be generated for providing an end of vector signal when said vector to be generated attains said length, said stroke state machine being responsive to said end of vector signal for entering said time pause state in response thereto, so as to insert a time pause between generating each of said vectors, and means coupled to said video input for blanking the video of said CRT means to coincidence with said time pause. 